The present invention relates generally to materials testing apparatuses, and more specifically, to an indenter for a nanoindentation instrument.
The use of penetration techniques to obtain information relating to the mechanical properties of a material sample is well known. Typically, an indenter having a indentation tip at the distal end is forced into a sample of a material and then extracted therefrom, leaving an indentation in the surface of the sample. Indicia pertaining to depth of penetration and force applied to the indenter are manipulated, typically by a dedicated computer, to determine material properties such as hardness or modulus of elasticity.
The testing of materials in this way can be readily performed in the nanoscale range through the use of several commercially available testing apparatuses. For example, MTS Systems Corporation, Eden Prairie, Minn. manufactures and sells a system for nanoindentation testing under the trademark Nano Indenter XP. Systems such as this one from MTS and others are quite effective at providing test results for small volumes of material (volume sizes less than 1000 cubic microns). Almost all nanoindentation systems use optical imaging to determine the location of the indentation test prior to testing, but this method is usually precise only to within a few microns. For ultra-precise positioning of the indentation tip within a few nanometers, some nanoindentation instruments (such as the Nano Indenter XP) can scan the indentation tip over the surface prior to testing, thereby creating a detailed image of the surface topology with nanometer-level resolution. This image can be subsequently used to precisely define the location for the indentation test.
These nanoindentation systems are not without their shortcomings, however, because the information gained from testing using a sharp indentation tip to penetrate a flat surface is generally limited to elastic modulus and hardness of the region tested. Information obtained from uniaxial compression experiments such as yield strength, ductility and work hardening would be desirable as well but can't be ascertained by traditional nanoindentation methods. Using micro-machining methods such as Focused Ion Beam machining, laser ablation, or Electrode Discharge Machining, compression samples with volume sizes less than 1000 cubic microns can be fabricated into the surface of a material and tested in uniaxial compression using the aforementioned commercial testing apparatuses. For uniaxial compression testing, the loading axis of the indenter tip must be positioned precisely parallel with the centerline of the compression specimen. If not, then bending moments may be applied to the test specimen that result in invalid test data. The requisite, precise placement of the indentation tip for uniaxial experiments using only an optical microscope is difficult or impracticable within the commercial nanoindentation apparatuses, but is easily achieved using the surface scanning/imaging method. However, these compression samples cannot be tested with a sharp tip, as the interpretation of the test data for a compression experiment relies on a uniform imposed stress state. A blunt or flat tip is used instead, but this prevents the use of the ultra-precise scanning/imaging technique, as the resolution of the system's scanning imaging capability is dependent on the shape of the indentation tip. As a result, the desirable uniaxial compression experiments have not been heretofore practicable.
A need exists therefore for an improved indenter tip for nanoindentation systems having a machined flat for uniaxial compression testing that can be precisely positioned. Such a tip would provide the dual in situ functionality of uniaxial compression loading and scanning/imaging of the sample surface.